The long-range goal of this work is to develop a comprehensive model of mechanical influence on craniofacial growth. As a first step, the current project will characterize postnatal growth and stress/strain relations in a system which is well understood functionally: the masseter-zygomatic arch complex in the pig. Questions to be addressed include: (1) Can bone growth and periosteal stretching account for ontogenetic changes in muscles function? This study will employ radiographic markers for simultaneous monitoring of muscle, periosteum, and bone growth. (2) How do functional strains in the zygomatic arch relate to activity in the masseter? In vivo bone strain will be measured simultaneously with muscle activity while pigs are chewing. (3) Does bond apposition follow the orientation of net or maximum strain? Growth data from (1) will be correlated will strain data from (2) to answer this question. (4) Is the growth of intramuscular tendons determined by muscle force? The growth of the superficial aponeurosis of the masseter will be analyzed quantitatively and correlated with muscle fiber orientation. (5) What are the in vivo strains in intramuscular tendons? Strain will be measured directly with cinefluorography and correlated with anatomical data. Finally, these studies will be generalized to other species to see whether the causal relationships hold true for other mammals (including man), and whether prenatal growth follows the same rules as postnatal growth. This work has direct application to clinical problems related to abnormal biomechanical effects, such as facial deformities and conseqences of orthognatic surgery.